1. Field of the Invention
The invention relates to an encoding circuit for a SECAM colour television transmitter, comprising a frequency-modulatable oscillator for converting a modulating signal which contains two line-sequential colour difference signals into a frequency-modulated signal which has a first quiescent frequency when the first colour difference signal is zero and a second quiescent frequency when the second colour difference signal is zero, also comprising a control loop for making the first quiescent frequency substantially equal to the frequency of a first reference signal generator and the second quiescent frequency substantially equal to the frequency of a second reference signal generator, this control loop comprising a multiplexer operated by a pulse generator for applying in a predetermined sequence the frequency-modulated signal and the first and second reference signals to a frequency demodulator to which is connected a first control path for generating a first control information for the oscillator and a second control path for generating a second control information for the oscillator, the first and second control paths each being operative during predetermined measuring intervals in which the signal conveyed by the multiplexer has the reference or the carrier frequencies, respectively, for comparing the said frequencies.
2. Description of the Related Art
Such an encoding circuit is disclosed in British Patent Specification No. 1,509,958. The operation of the control loop in this prior art circuit ensures that the frequency of the frequency-modulated signal is equal to the frequency of the relevant reference signal generator during a time interval in which the modulating colour difference signal is zero. This is effected line-sequentially, that is to say for one line the quiescent frequency of the frequency of the red reference signal f.sub.OR =4.40625 MHz and for the subsequent line it assumes the frequency of the blue reference signal f.sub.OB =4.250 MHz. As the frequency-modulated signal must be at its quiescent frequency during the line blanking period, this signal is demodulated for a short period within this blanking period and the information thereby obtained is compared with the information obtained by demodulation of the reference signal during the subsequent line scanning period. The error signal thus obtained, corresponding to the difference between the quiescent and reference frequency contains the required control information. For this purpose the control loop comprises two separate control paths, one for red and one for blue.
In the prior art circuit the frequency demodulator receives its input signal from a multiplexer in a predetermined sequence. During a line blanking period the frequency-modulated signal with the frequency f.sub.R generated by the oscillator is conveyed to the demodulator and during the subsequent line scan period the signal with the reference frequency f.sub.OR is conveyed to the demodulator. During the next blanking period the frequency-modulated signal f.sub.B and during the subsequent line scanning period the reference frequency signal f.sub.OB are conveyed to the demodulator's whereafter the sequence is repeated. So within a sequence of two lines, the signals to be compared succeed each other after the short period of time between a blanking period and the next line scan.
This sequence has the following drawback. A square-wave switching signal of half the line frequency is operative in the encoding circuit in several places, more specifically for changing-over the information from a blue line to the information of a red line and vice versa. This change-over is usually effected during the line blanking period, i.e. at an instant at which no colour signals are generated. For the prior art encoding circuit this means that the change-over occurs during a very short period of time, namely some micro-seconds, prior to measuring the frequencies f.sub.R and f.sub.B. However, at the change-over a very rapid change of the frequency occurs, which change produces a large and sudden phase change for the frequency demodulator. At the measuring instant this sudden change may influence the level to be measured, which introduces an error in the control information applied to the oscillator.